1. Field of the Invention
The present invention relates to a method and an apparatus for processing an image, and more particularly to a method and an apparatus for processing a binary image.
2. Description of the Related Art
Generally, a scanner is configured as follows. Light is projected onto figures drawn or printed on paper or photographs, and light reflected thereby is received by a light-receiving element such as a CCD (Charge-Coupled Device). Then, the tone of color is converted into the intensity of light for digitization.
An image (halftone dot image) of a binary figure or photograph is printed so that a large number of halftone dots consisting of a plurality of dots are formed on paper at a given pitch. A monotone image is printed so that a large halftone dot (consisting of a comparatively large number of dots) is arranged for a dark portion of the monotone image, and a small halftone dot (consisting of a comparatively small number of dots) is arranged for a light portion thereof.
Generally, the scanner reads a binary document in which dot images, line drawings and characters coexist as an aggregate of fine dots. A (input) resolution indicates how finely the document can be read.
In newspaper corporations and desktop publishing (DTP) corporations, when a photographic printing paper or a binary document transmitted from a remote place such as a facsimile image is optically read by the scanner, it is desired to input images in an image size that matches the sheet insertion size. In other words, it is desired to input images at a desired resolution.
However, in practice, the scanner can read the binary document only at a fixed resolution because of a restriction on hardware. Thus, the binary document is once input at the fixed resolution, and the image size thereof is changed, so that a desired binary-valued document can be generated.
However, in that case, a chess-board like distortion may occur when input binary data is decimated at irregular intervals or is changed in size for enlargement or reduction.
An improved technique taking the above into account has been proposed. This technique does not change the document size based on input binary data of the document. Instead, the proposed technique inputs multi-valued data of the binary document so as to match the sheet insertion size (the gray scale of the halftone dot is divided into, for example, 256 gradation levels). Then, the multi-valued data is converted into binary data in a post process. The proposed technique prevents occurrence of a chess-board like distortion resulting from decimation and change of size.
In the proposed technique, a binary image is generated by simple binarization (in which a threshold value of 127 is used for 256 gradation levels). However, simple binarization frequently degrades images, which may include a Moiré that occurs in a halftone dot image or a jaggy noise that occurs in a line drawing or a character (a portion that should be originally a straight line has a rough contour).
Conventionally, simple binarization is employed without definitely separating a halftone dot image part and a line drawing/character part from each other. However, it is desired that the halftone dot image part and the line drawing/character part are processed by logically different manners in order to obtain high-quality output.
Thus, even if improvement in the image quality is attempted by simultaneously employing a Moiré smoothing process as measures against Moiré of halftone dot images and a contour emphasizing process as measures against the jaggy noise in the line drawing/character part, these processes may affect each other, and the expected effects would not be obtained.